1. Field of the Invention
The present invention relates to a self-propelled crane of telescopic boom type and provided with additional boom means to improve the lifting capacity of the crane and increase the possibilities for its use in limited working spaces. A greater lifting capacity is needed, for example, in situations wherein the maximum allowed weight of the load to be lifted is limited by either the risk of the crane to tip over or by the loading values of some structural part. Increased possibilities for using the crane in limited working spaces are needed, for example, in situations wherein an obstacle below the telescopic boom prevents lifting to a great distance owing to the limited lowering possibilities.
2. Description of the Prior Art
Previously known are additional booms which are installed at the upper end of the topmost boom part, either rigidly without supporting hoists or journal bed and supported by wires or hydraulic elements, for example.
These additional booms have a disadvantage in that the positions of the additional boom and the supporting wires cannot be easily adjusted to suit each lifting task from the operator cab by means of the telescopic boom. Another disadvantage is the great bending moment the telescopic boom will be exposed to. Due to the big stresses the maximum lifting capacity achieved within a certain distance is considerably less with telescopic booms provided with an additional boom installed at the upper end of the telescopic boom than the lifting capacity achieved within the same distance with the same crane provided with additional booms according to the invention.